We need to remove the envelope
so we can modify the fills. Select the enveloped checkerboard, then select Convert to Editable Shapes from the Arrange menu.
Ungroup (Ctrl - U) the checkerboard twice.
Select the large square and apply a Linear Fill using the two shades of black as shown (10-40%).
Select the checkers and apply a Linear Fill using the two shades of black as shown (100-70%).
Making the light and dark colors closer in value as they go to the back, enhances the feeling of depth.
Make a duplicate of the right side
and rotate the duplicate counter clockwise 90 degrees to make the ceiling.
We need to remove the envelope
so we can modify the fills. Select the enveloped checkerboard, then select Convert to Editable Shapes from the Arrange menu. 
Make a duplicate of the right side
and rotate the duplicate counter clockwise 90 degrees to make the ceiling.